The present invention relates to a method of creating a weld to connect a first element to a second element. For a variety of applications it would be desirable to provide a method of creating a weld in a wellbore formed in an earth formation, the wellbore containing a wellbore fluid. Such applications include, for example, forming of metal-to-metal seals, providing structural integrity to wellbore components or repairing corrosion damage. However welding in a liquid environment (the wellbore fluid) at high pressures generally encountered in wellbores, is not feasible. An analogous situation exists for underwater welding, e.g. at offshore platforms, whereby the depth at which such welding is feasible is limited by the hydrostatic pressure of the water.
It is an object of the invention to provide a method of creating a weld to connect a first element to a second element, said elements being arranged in a wellbore containing a wellbore fluid.
The method according to the invention comprises
a) selecting a welding path along which the weld is to be created;
b) selecting a volume portion of the wellbore, in which volume portion said path is located, and sealing said selected volume portion from the remainder of the wellbore volume;
c) providing pressure control means for controlling the fluid pressure in said selected volume portion;
d) operating the pressure control means so as to reduce the fluid pressure in said selected volume portion to a selected pressure at which the weld can be created; and
e) creating the weld along the selected welding path.
By sealing the volume portion in which the weld is to be created from the remainder of the wellbore, and subsequently reducing the pressure in the volume portion, the fluid pressure no longer prevents the creation of the weld.
Suitably the first element is an upper wellbore casing and the second element is a lower wellbore casing having an upper part extending into a lower part of the upper wellbore casing. In this manner metal-to-metal sealed casings are obtained allowing gas at high pressure to be produced through the wellbore casing rather than through a conventional production tube extending through the wellbore casing.
In a preferred embodiment said upper part has an outer diameter substantially equal to the inner diameter of said lower part, and step e) comprises welding the upper edge of the lower casing to the upper casing.
In another preferred embodiment said upper part has an outer diameter smaller than the inner diameter of the lower part, and wherein the method further comprises inserting a filler tube in the annular space between said upper part and said lower part, and wherein step e) comprises welding the upper edge of the lower casing to the filler tube and welding the upper edge of the filler tube to the upper casing.
To allow high pressure gas production from a multilateral wellbore system, suitably the wellbore is a main wellbore provided with a branch wellbore, said first element being a branch of a branch casing member which connects a main casing extending through the main wellbore to a branch casing extending through the branch wellbore, said second element being the branch casing.